How to Do a Long-Tail Cast On
The long-tail cast on, also called the thumb method or the slingshot method, produces a very elastic edge. It is particularly useful when followed by garter stitch or stocking stitch.
Check the number of stitches the pattern requires to be cast on. For size 4 (worsted/medium) weight yarn, measure approximately 1" of yarn for each stitch and make a slip knot at this point. For thicker yarn, allow more yarn for each stitch. For example, if the pattern calls for you to cast on 100 stitches, pull out approximately 100" of yarn. For this method of casting on, it is always better to err on the side of pulling out too much yarn rather than too little.
Hold the needle with the slip knot in your right hand, and hold the ball end of the yarn parallel to the needle. * Wind the loose end of the yarn around your left thumb from front to back.
Insert the needle through the yarn on your thumb.
With your right index finger, wrap the ball end of the yarn over the needle point.
Pull a loop through to form the first stitch.
Remove your left thumb from the yarn and pull the loose end to tighten the stitch against the needle. Don't make the stitch extremely tight, as you will need to put your needle through the stitch when you start knitting.
Repeat from * until the required number of stitches has been cast on.